The present invention relates to a power supply circuit, a display driver, an electro-optic device and an electronic apparatus.
An active-matrix liquid crystal display device has a plurality of scanning lines and data lines formed in matrix. It also has a plurality of switching elements, each of which is connected to a scanning line and a data line, and a plurality of pixel electrodes, each of which is connected to a switching element. The pixel electrodes are facing the counter electrodes through liquid crystal (in a broader sense, electro-optic material).
In a liquid crystal display device with such a structure, a voltage, supplied to the data line through the switching element activated by the selected scanning line, is applied to the pixel electrode. Then, corresponding to the voltage applied between the pixel electrode and the counter electrode, the transmittance of the pixel changes.
The liquid crystal in liquid crystal display devices needs to be driven in alternating current, so as to prevent the deterioration thereof. Therefore, a polarity inversion drive is performed in liquid crystal display devices, in which the polarity of the voltage between the pixel electrode and the counter electrode is inverted once per frame or at least once per horizontal scanning period. The polarity inversion drive is implemented by, for example, changing the voltage supplied to the counter electrode in synchronization with the polarity inversion timing.
Using operational amplifiers is an example of implementing the aforementioned polarity inversion drive by supplying a boosted voltage to the counter electrode with its charge pump operation.
JP-A-2002-366114 is an example of related art.
In the active-matrix liquid crystal display device, liquid crystal is inserted between the pixel electrodes and the counter electrodes, and the pixel electrodes and the counter electrodes are linked with capacitance component. Therefore, when the voltage supplied to the data line is applied (written in) to the pixel electrode via the switching element selected by the scanning line, the voltage level of the counter electrode changes at the time of voltage application, along with voltage fluctuation of the pixel electrode.
In this case, the operational amplifier can bring the voltage level of the counter electrode back to its original value at the write-in time of the pixel electrode, by increasing the output capacity of the operational amplifier (slew rate and electric current drive capacity). However, the output capacity enhancement of the operational amplifier results in an increase in power consumption.
In recent years, there has been a study to make the pixel finer and to attain a smaller display panel size, by forming a display panel (in a broader sense, an electro-optic device) typically a Liquid Crystal Display (LCD) panel with a Low Temperature Poly-Silicon (hereafter LTPS) process (a type of manufacturing process). With the LTPS process, a part or all of the driving circuit of the display panel can be formed directly on the panel substrate (for instance, a glass substrate), on which the pixel including the switching element (for instance, a Thin Film Transistor, or TFT), is formed.
For instance, a display panel utilizing the mobility of electric charge in the LTPS, may be provided with a demultiplexer that connects one data signal supply line to any one of the data lines for R, G, and B components; where, a data signal (drive voltage) is supplied to the data signal supply line, and the data lines for R, G, and B components can be connected to the pixel electrode of the R, G and B components (first through third color components composing one pixel). In this case, a multiplexed signal, into which the data signals for the R, G, and B components are handled with time-division, is supplied to the demultiplexer. Thereafter, the data signals for the color components are sequentially switched and output into the data lines for the R, G, and B components by the demultiplexer, and written in to the pixel electrode provided for each color component. Such configuration allows a reduction of the number of terminals for outputting the data signal to the data line from the driving circuit. Therefore, it is possible to cope with the data line number increase, which is the result of pixels fining, without being limited by a narrow pitch between the terminals.
However, in the case of driving the display panel provided with such a demultiplexer, the write-in time to the pixel electrode becomes all the more shorter, in comparison to the case of driving a normal display panel. Accordingly, it is necessary to further shorten the time of bringing the fluctuated (as described above) voltage level of the counter electrode back to its original value. In order to shorten the time, the output capacity of the operational amplifier that drives the counter electrode needs to be enhanced all the more, resulting in a further increase in power consumption.